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<p>
While transactions are frequently used to provide atomicity to
multiple database
operations, it is sometimes necessary to perform
a single database
operation under the control of a transaction.
Rather than force you to obtain a transaction, perform the single
write operation, and then either commit or abort the transaction,
you can automatically group this sequence of events using
<span class="emphasis"><em>auto commit</em></span>.
</p>
<p>
To use auto commit:
</p>
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<li>
<p>
Open your environment and your databases
so that they support
transactions. See <a href="enabletxn.html">Enabling Transactions</a>
for details.
</p>
<p>
Note that frequently auto commit is used for the environment
or database open. To use auto commit for either your
environment or database open, specify
<tt class="literal">DB_AUTO_COMMIT</tt> to the
<tt class="methodname">DbEnv::set_flags()</tt>
or
<tt class="methodname">Db::open()</tt>
method. If you specify auto commit for the environment
open, then you do not need to also specify auto commit
for the database open.
</p>
</li>
<li>
<p>
Do not provide a transactional handle to the method that is
performing the database
write operation.
</p>
</li>
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<p>
Note that auto commit is not available for cursors. You must always
open your cursor using a transaction if you want the cursor's
operations to be transactional protected. See
<a href="txncursor.html">Transactional Cursors</a> for details on using
transactional cursors.
</p>
<p>
For example, the following uses auto commit to perform the database write operation:
</p>
<pre class="programlisting">#include "db_cxx.h"
...
int main(void)
{
u_int32_t env_flags = DB_CREATE | // If the environment does not
// exist, create it.
DB_INIT_LOCK | // Initialize locking
DB_INIT_LOG | // Initialize logging
DB_INIT_MPOOL | // Initialize the cache
DB_INIT_TXN; // Initialize transactions
u_int32_t db_flags = DB_CREATE | DB_AUTO_COMMIT;
Db *dbp = NULL;
const char *file_name = "mydb.db";
const char *keystr ="thekey";
const char *datastr = "thedata";
std::string envHome("/export1/testEnv");
DbEnv myEnv(0);
try {
myEnv.open(envHome.c_str(), env_flags, 0);
dbp = new Db(&myEnv, 0);
// Open the database. Note that we are using auto commit for
// the open, so the database is able to support transactions.
dbp->open(NULL, // Txn pointer
file_name, // File name
NULL, // Logical db name */
DB_BTREE, // Database type (using btree)
db_flags, // Open flags
0); // File mode. Using defaults
Dbt key, data;
key.set_data(keystr);
key.set_size((strlen(keystr) + 1) * sizeof(char));
key.set_data(datastr);
key.set_size((strlen(datastr) + 1) * sizeof(char));
// Perform the write. Because the database was opened to support
// auto commit, this write is performed using auto commit.
db->put(NULL, &key, &data, 0);
} catch(DbException &e) {
std::cerr << "Error opening database and environment: "
<< file_name << ", "
<< envHome << std::endl;
std::cerr << e.what() << std::endl;
}
try {
if (dbp != NULL)
dbp->close(0);
myEnv.close(0);
} catch(DbException &e) {
std::cerr << "Error closing database and environment: "
<< file_name << ", "
<< envHome << std::endl;
std::cerr << e.what() << std::endl;
return (EXIT_FAILURE);
}
return (EXIT_SUCCESS);
} </pre>
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